A role for free radicals has been postulated for a number of pathological conditions, especially cancer, but studies of complex biological materials has been hindered by the inability to characterize spectra of immobilized free radicals. It is proposed to develop a methodology for differentiating, characterizing and identifying the immobilized free radical signals in biological systems. A catalog of multi-ESR displays will be made for free radicals in biological systems. The catalog will include (i) conventional ESR spectra at X, Q and S-band, (ii) eight non-conventional displays at high power (in and out of phase for absorption, dispersion, 1st and 2nd derivative), (iii) computerized difference or stripped spectra, and (iv) new displays (pulse spectroscopy and cross correlation filtering). The physical basis of identification depends on differences in both the magnitude and orientation of the g- and hyperfine tensors, the mixing of nuclear states and forbidden transitions, different relaxation parameters, and passage effects. Model free radicals to be studied include a pyrimidine, flavin, phenothiazine, semiquinone, vitamins B, D, and E, an immunosuppressant, free radical scavenger, and a tumor inhibitor. The model system study of immobilized free radical signals should be the foundation for understanding the origin of free radical signals particularly in tumor as well as other biological systems.